8/8 DESIGN FOR A BRAIN 



(J£), which determines the polarity of entry to the coil, and 

 through a potentiometer (P), which determines what fraction of 

 the input shall reach the coil. 



As soon as the system is switched on, the magnets are moved 

 by the currents from the other units, but these movements change 

 the currents, which modify the movements, and so on. It may 

 be shown (S. 19/11) that if there is sufficient viscosity in the 

 troughs, the four-variable system of the magnet-positions is 

 approximately absolute. To this system the commutators and 

 potentiometers act as parameters. 



When these parameters are given a definite set of values, the 

 magnets show some definite pattern of behaviour ; for the para- 

 meters determine the field, and thus the lines of behaviour. If 

 the field is stable, the four magnets move to the central position, 

 where they actively resist any attempt to displace them. If 

 displaced, a co-ordinated activity brings them back to the centre. 

 Other parameter-settings may, however, give instability ; in 

 which case a c runaway ' occurs and the magnets diverge from 

 the central positions with increasing velocity. 



So far, the system of four variables has been shown to be 

 dynamic, to have Figure 4/12/1 (A) as its diagram of immediate 

 effects, and to be absolute. Its field depends on the thirty-two 

 parameters X and P. It is not yet ultrastable. But the inputs, 

 instead of being controlled by parameters set by hand, can be 

 sent by the switches S through similar components arranged on 

 a uniselector (or ' stepping-switch ') U. The values of the com- 

 ponents in U were deliberately randomised by taking the actual 

 numerical values from Fisher and Yates' Table of Random 

 Numbers. Once built on to the uniselectors, the values of these 

 parameters are determined at any moment by the positions of 

 the uniselectors. Twenty-five positions on each of four uni- 

 selectors (one to each unit) provide 390,625 combinations of 

 parameter- values. In addition, the coil G of each uniselector is 

 energised when, and only when, the magnet M diverges far from 

 the central position ; for only at extreme divergence does the 

 output-current reach a value sufficient to energise the relay F 

 which closes the coil-circuit. A separate device, not shown, 

 interrupts the coil-circuit regularly, making the uniselector move 

 from position to position as long as F is energised. 



The system is now ultrastable ; its correspondence with the 



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